1. Field of the Invention
The present invention relates generally to devices which sense the position of a rotatable member and, more particularly, to a rotary position sensor that is suitable for use as a geartooth sensor or a slot sensor and which can provide information related to the speed or angular position of a rotatable member.
2. Description of the Prior Art
Many different types of sensors have been developed over the years to determine the angular position of a rotatable member. For example, sensors are very well known in the prior art for determining the rotational speed and direction of a gear having a plurality of teeth. In addition, sensors have been developed for determining the rate of rotation of a shaft, such as a camshaft, which is provided with a single discontinuity, or notch, in the outer periphery of the shaft.
U.S. Pat. No. 4,745,363, which issued to Carr et al on May 17, 1988, discloses a geartooth sensor which utilizes a Hall cell. It is intended for use in sensing gearteeth or similarly shaped discontinuities in a ferrous magnetic or magnetic target. It utilizes a single magnet and a direct coupled Hall cell integrated circuit to detect the presence or absence of gearteeth. It comprises spaced apart offset flux concentrators to provide a sensitive device with favorable tolerances in temperature and air gap.
U.S. Pat. No. 4,926,122, which issued to Schroeder et al on May 15, 1990, describes a high sensitivity magnetic circuit in which the stationary portion includes a permanent magnet whose width is optimally 1.5 times the tooth pitch of the exciter portion of the sensor and the magnet face proximate the exciter includes a thin layer of ferromagnetic material over which is centered a narrow magnetic sensing element, such as a magnetoresistor. The sensing element has a width typically less than the tooth width which is between 0.17 and 0.37 times the tooth pitch. The needed magnetic flux density is typically available simply by appropriate magnet thickness or choice of magnet material without the need of a flux guide.
U.S. Pat. No. 4,972,332, which issued to Luebbering et al on Nov. 20, 1990, discloses an apparatus for determining the speed, angular position and direction of rotation of a rotatable shaft. The apparatus is intended for use on an electronically controlled fuel injection system. It uses a single Hall effect sensor. A disk element is fixedly connected to and rotatable with the cam shaft of an internal combustion engine. The disc element includes a plurality of circumfrential zones of substantially identical length with each zone having first and second areas. A first portion of these circumfrential zones has first and second areas which are substantially different in length than the first and second areas of a second portion of the circumfrential zones. Therefore, the sensor delivers a signal which has a frequency directly related to the instantaneous velocity of the disc element, but varies in pulse width in response to the first and second portions of the circumfrential zones.
U.S. Pat. No. 4,853,632, which issued to Nagano et al on Aug. 1, 1989, describes an apparatus for magnetically detecting a position of a moveable magnetic body. It includes a three terminal magnetic field intensity sensing structure formed by a pair of magnetic resistors. The magnetic field intensity sensing structure is disposed opposite to a magnetic body arranged for movement relative thereto in a magnetic field and generates a first electrical signal of sinusoidal waveform in response to a change in the intensity of the magnetic field due to the relative movement of the magnetic body. This first electrical signal appears from the apparatus as a second electrical signal of rectangular waveform or a sinusoidal waveform having an amplified amplitude. Components of the circuit for shaping the waveform or amplifying the amplitude of the first electrical signal are integrally mounted together with the magnetic resistors on a substrate. Preferably, the shaping or amplifying circuit is in the form of a hybrid integrated circuit formed on the substrate.
Position sensors of the type described above typically exhibit several disadvantages when used in certain applications. For example, it is often difficult to accurately detect the speed of a rotatable member if only a single discontinuity is available in the surface of the member. In addition, upon initial start up of the system incorporating a position sensor, it is often difficult to precisely determine the position of the sensor relative to the rotatable member without first rotating the rotatable member.